Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 031512

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1012 AM CDT Wed Jun 3 2020

1012 AM CDT

Showers and thunderstorms continue to fester across west-central
Illinois close to a low-level moisture reservoir and zone of
modest low- level confluence. So far this morning, all convective
cells have weakened once reaching roughly I-39 thanks to meager
instability over northeastern Illinois. We continue to see the
occasional lightning flash out our window as apparently necessary
charge separation remains in a somewhat composite stratiform

In the wake of the morning activity, focus will turn to the
potential for renewed convective development ahead of a weak
surface low currently in central Iowa. As is always the case with
morning convection, identifying areas of renewed development is
a fun challenge (honestly won`t even look at the CAMs), but do
think the better chance will be along residual outflow boundaries
or in proximity to ongoing convection in central Illinois (perhaps
as far south of State Highway 24). As such, have cautiously
trimmed PoPs areawide and confined the highest values toward the
central Illinois. Also, with continued cloud cover, decided to
drop high temperatures south of I-80 to the lower 80s, with areas
perhaps along the IL/WI state line having the best shot of
reaching the mid-80s with less cloud cover this afternoon.

Updated products will be sent soon.



325 AM CDT

Through Tonight...

Thunderstorm chances and coverage today were the emphasis in
our forecast analysis this morning. While there will be residual
foci for storms in the form of outflows and an incoming front
within a moist, unstable atmosphere, there is also weak forcing
and that presents a fair amount of uncertainty for today`s
forecast, though some storms in the area are a good bet. Heavy
downpours in any storms will be almost a guarantee, so if any
corridors of temporary training storms develop there would be some
flood threat. Also a scattered severe threat in the form of
marginally severe hail and localized downbursts remains with any
sustained mature storms. The area for these concerns look to be
for south of I-88 from late morning into the afternoon.

Satellite and radar imagery as of 3 a.m. portrays a corridor of
convection across the IL/WI state line back to Waterloo, IA. This
is within a zone of 700-850 mb convergence and assisted in the
upper levels by a broad right entrance region of a 100 kt jet
maximum located in central Michigan. The convection also likely
has some support of an ill-defined mesoscale convective vortex
(MCV) near the IA/IL/WI border region. The activity has had
difficulty moving southward or redeveloping new updrafts south
due to a veered low-level jet and steering flow that has more
readily taken convection eastward. Expecting through 9 a.m. or so
that the current thunderstorms show a gradual decline, sustaining
more so on MUCAPE around 1,000 J/kg and PWATs of 1.7 inches than
strong forcing.

In the low-levels to boundary layer (i.e. 850 mb to surface), a
weak cold front is draped near to just north of the convection
this morning that is slowly oozing southward. A 1008 mb low near
Des Moines as of 3 a.m. will evolve eastward along this boundary
in response to the early mentioned upper forcing. Convergence
along the boundary in the low-level should improve during the
later morning hours. Any mid-level impulse or yet another MCV is
challenging to decipher in guidance so the impetus looks to be
mainly just convergence forcing moist parcels to the level of free
convection. The CAM guidance trends have been more onto springing
later morning convection near the I-88/290 corridor where this
focus will be. Think some of the coverage and magnitude of this
may depend on how much the current activity diminishes. If it
continues to fester that may keep any redevelopment somewhat
muted. There would be some marginal severe threat with a few
storms, and some flooding threat given the west-to-east nature and
the possibility they will drift over the Chicago metro.
Confidence in this 9 am - 1 pm time frame is pretty low.

As the composite front continues to sink south into the peak
heating hours of afternoon, thunderstorms chances will increase
south of I-80. Dew points look to be solidly mid 60s and possibly
upper 60s, supporting mixed layer CAPE of 1500-2500 J/kg. There
may be some residual capping if cloud cover from the north
becomes more solid than expected over the southern CWA. While
confidence is high in some thunderstorms for south of I-80 during
the afternoon, it`s uncertain on the coverage and depends some if
the earlier mentioned late morning activity to the north
develops. Deep layer shear looks to be 20-30 kt which is marginal
for severe, but the instability from 7-7.5 C/km mid-level lapse
rates and the high PWAT air yield the potential for isolated to
scattered downburst winds and severe hail. Slower storm motions
along the boundary may mean at least a localized corridor of flood
concern too, and that might be the bigger issue, but again
depends on mode evolution and coverage. Unfortunately, confidence
on that remains low during the P.M. The convection is likely to
shift out of the area by mid-evening.

Otherwise for today, temperatures will likely have some
fluctuation under the clouds and any temporary convective
cooling. Highs look to be lower to mid 80s inland, possibly upper
80s south. Winds lakeside will turn onshore by afternoon but may
be due to outflow this morning, and this will keep lakeside areas

For later tonight, fog may be possible especially near the
immediate lake shore and south of I-80 where clearing looks to
arrive near sundown with residual high dew point air.



316 AM CDT

Thursday through Tuesday...

Above normal temperatures continue to be favored into early next
week with highs generally in the 80s for inland areas. Cooler
temps are expected near the lake Thursday and again this weekend
thanks to light northeast flow.

Any lingering patchy fog from overnight Wednesday will dissipate
by morning. Precip chances Thursday will be highly dependent on
how things ultimately unfold on Wednesday and where the resultant
outflow boundary/cold front stalls out. Continue to favor models
that show this being south of our area. So have only maintained
slight chances for our far southern counties on the off-chance
that the boundary lifts back north far enough. Otherwise expect
things to be mostly dry across the area with highs in the mid to
upper 80s. A lake breeze is expected to move inland Thursday
afternoon which should keep highs near the lake in the 70s.

By late Thursday night into Friday a weak wave is expected to
traverse along the zonal upper level flow bringing additional
thunderstorm chances to the area. There still remains plenty of
model variability on the placement of convection given the weaker
forcing in place. If convection is able to develop in our area
there will be sufficient instability and shear in place by Friday
afternoon for storms to become strong to possibly severe. Another
warm and humid day is expected with highs in the mid to upper 80s
and dewpoints in the upper 60s to near 70.

The upper level ridge then builds in for the weekend with a
Canadian surface high sinking south helping turn winds
northeasterly off the lake. This should help cool things down
somewhat especially for areas near the lake. Still expecting low
to mid 80s inland. Drier air should also make things feel a bit
more pleasant than the past several days.

Heading into early next week the ridge will begin to shift
eastward as a trough approaches from the west. This combined with
possible interactions with the tropical system in the gulf
indicate precip chances will be on the increase by mid week. It is
still too far out to hammer out specifics, however.



For the 12Z TAFs...

Main concerns through the TAF period:

* Timing and coverage of thunderstorms this morning into early
* Convective influence of wind directions and gusts this morning
* Lake breeze timing this afternoon

A narrow east-west oriented axis of showers and thunderstorms
continues to drift east southeast across northern Illinois this
morning. Plenty of uncertainty remains with how things will unfold
even within just the next few hours. Have added a VCTS mention to
start the TAF at ORD and MDW to account for the persistent
lightning embedded in the thunderstorm cluster moving east across
northern Illinois.

Hi-res model guidance continues to indicate additional
thunderstorm development will try to develop along the I-80
corridor by mid morning, so have maintained the TEMPO TSRA from
16-18Z for ORD & MDW to account for this potential. However,
confidence in this remains low. Winds then turn northwesterly
behind the front as it moves through. Any lingering thunderstorms
are expected to drop south of area terminals by 19-20Z.

A lake breeze is also expected this afternoon, the exact timing
of which may be complicated somewhat by any resultant thunderstorm
outflows. Once it is able to push through, winds will turn
northeasterly to near easterly across the Chicago area terminals
this afternoon. Winds become mostly light and variable area wide
this evening through tomorrow morning.





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